It is my opinion that emeralds are the hardest stone to judge in the rough and to cut; harder than any other colored stone. Emeralds from any source seldom reveal their true clarity, color or transparency until they are cut and polished.

I also feel that the Colombian emerald cutters are the best emerald cutters in the world. They face daily things like zoning, inclusions and fissures – as well as bosses who want the most beautiful stone possible with the least amount of weight loss. Many times a piece of rough that I bring to Tucson to sell to the cutters there ends up going back to Colombia where I invariably get more money selling in the downtown Bogotá emerald market; they know the material the best and know how valuable the resultant gem will be.

In good overhead lighting, the African emeralds faceted by the Colombian cutters show off their great color. For more discussion about the optical and color differences between African and Colombian emeralds, please see Chapter Five of Emeralds, A Passionate Guide.

Here are the things learned as we took the African emeralds through the cutting process in downtown Bogotá.

[Disclaimer: the parcel of African stones was rather low-end. The conclusions and opinions only apply to this material – not all emeralds from Africa.]

The African material is bluish for sure. Almost as blue as Colombian euclase (which is found in the Andes mountains in the emerald mining region). The next thing they said was “not bad!” After all, bluish is better than yellowish in emeralds. They saw potential in the material.

The rough consisted of 300 stones averaging a gram each. We expected a yield of stones from .50 ct. to 2 carats. The quality was mixed which pleased the cutters: they will have cabochons to cut as well as commercial and nicer faceted stones.

It was remarkable how, in a parcel of almost 300 stones the Colombian cutters immediately picked out the five or six best rough emeralds. Their hands went to them immediately!

The ever-present black inclusions in the African material (inclusions of dark brown biotite flakes and rutile rods) quickly became a concern. The Colombian cutters gave it a name: “café molida” or ground coffee.

Many of the African emeralds, after faceting, had dark zones and black areas that made them look unappealing. The application of oil treatment however brightened the stones up quite nicely.

Black spots in African emeralds; Part Two: The arrows on the left image point to small horizontal lines that look inconsequential until the emerald is tilted. When tilted it is revealed that we were looking end-on at biotite flakes. The cutters had oriented the flakes perpendicular to the table facet in order to reduce their visibility. They called those inclusions "ground coffee."

The Colombians use nitric and hydrochloric acids to clean calcite and iron stains off of the rough emerald material. Their big hope was that the same use of acid would clean the black inclusions out of the African emeralds but it did not.

After the first few stones were cut by the faceters in Bogotá, they became aware of the directional nature of the biotite flakes. If those flakes could be position perpendicular to the flat plane of the table their visibility could be reduced considerably (see picture above).

An emerald is not finished until the visible fissures are hidden with oil or polymer treatment. Much discussion was heard over how best to enhance the stones. Despite the reputation that African stones have of being clean and often not needing enhancment, it was obvious that not even the best stone in this parcel would enter the market without ”oiling.”

Perhaps the higher grades of African material are cleaner. This parcel however, which will sell on the wholesale market for $30 to $500 per carat, needed oil enhancement with cedarwood oil (actually a mixture of cedarwood oil and a non-hardening synthetic resin) to look “sellable.” The improvement from clarity enhancement is usually small but it makes a difference in ‘sellability’. It’s like waxing your car.

Ten days proved too little time to cut 300 stones. Half of the emeralds stayed in Colombia two weeks more for the cutting process to be completed.

If the emerald from Africa is tilted to catch the light at the right angle, the biotite flakes will reflect light and look like birch leaves in the fall. Sometimes the biotite will form long ribbons as seen on the Left.